Arrays of individually addressable light emitting diodes have been found to be useful in exposing film used in sophisticated camera systems of the type used in aircraft or orbiting satellites for providing highly resolved photographs of the earth's surface and objects thereon. In such systems the image seen by the camera is converted to digital form. Each data bit which is representative of a picture element or pixel is utilized to energize selected ones of a large number of light emitting diodes disposed in arrays. The film strip is transported past the arrays of light emitting diodes and is exposed by the light from the energized light emitting diodes. This results in highly resolved photographs reproducing in crisp detail very minute objects.
One problem associated with such systems is the accurate measurement of light intensity of the light emitting diodes to assure that the film is being properly exposed, i.e., not over exposed or under exposed. Conventional light monitors utilizing external optics have been found to be inadequate for measuring light intensity of the light emitted from the diodes. For example, use of reflectors disposed in the light path of the diodes to reflect the light to a conventional light monitor have been found to be impractical due to the large number and close spacing of the diodes.